Mixed signal generation circuits capable of generating both digital and analog signals are useful in a variety of applications, including generating test signals for transmission to various circuit elements. Traditional mixed signal circuit testers generate digital and analog signals using two distinct types of signal sources. Digital signals are typically produced by a digital subsystem and associated digital pin card electronics. Such a system typically comprises a digital vector memory coupled to a one-bit digital-to-analog converter, which essentially serves as an amplifier. Analog signals are generally produced by separate analog instruments, such as arbitrary waveform generators.
A typical arbitrary waveform generator may comprise a bank of memory storing a set of waveform samples. These samples are passed to a multi-bit digital-to-analog converter, which generates an analog signal from the samples received. A problem with this method when employed for on-chip self testing is that implementing the multi-bit digital-to-analog converter requires substantial substrate area. In large testers or on a chip, cost constraints generally prohibit implementing large numbers of these analog instruments, ultimately limiting the volume of mixed test signals produced. The limited analog resources often lead to longer test times, since the inputs must be stimulated one at a time.
Another approach to generating mixed signals is to generate a bitstream with a ring oscillator, and to apply the bitstream to a sigma delta modulator to create a noise-shaped sample. The noise shaped sample is then applied to a one-bit digital-to-analog converter. The output of the digital-to-analog converter is then filtered to form a smooth analog waveform. A problem with this approach is that the resulting analog signal is limited to 20 decibels below full scale. Devices under test typically require full analog stimuli, limiting the value of this approach. Another problem with this method is that the sigma delta modulator hardware is expensive to implement.